1. Field of the Invention:
This invention relates to a laser beam printer for printing a plurality of colors and a semiconductor laser device suitable for a light source thereof.
2. Description of the Related Art:
A laser beam printer for printing plural colors has a problem in that when the printing is performed using optic systems independent of each of plurality of colors, the respective colors are shifted in printing position, thus causing the decline in the quality of an image. As a printing method for a color printer, which is free of the occurrence of the problem on such color printing-position shifts, a method is known which is called tri level printing disclosed in U.S. Pat. No. 4,078,929, for example. This is a method of printing toner of different colors through the same optic system, using the ability to select charge polarity represented in the form of two positive and negative types as toner corresponding to colored particles. The principle of its printing is as follows:
The surface of a photoconductor is first charged by a charger. When the photoconductor charged with electricity by the charger is exposed with an intensity-modulated laser beam, the electric charge is removed according to the intensity of the laser beam and the distribution of electric potentials shown in FIG. 2 is formed on the surface of the photoconductor. When such a surface-potential distribution occurs in the surface of the photoconductor, first negatively-charged toner 119 is apt to adhere to a high-potential region 116 corresponding to a portion not exposed with the light or weakly exposed with the light. Neither the negatively-charged toner 119 nor positively-charged toner 120 adheres to an intermediate-potential region 117 formed by the medium exposure of light. On the other hand, the positively-charged toner 120 adheres to a low-potential region 118 strongly exposed with light by its own static electricity. When the intermediate-potential region and the low-potential region are plotted on the photoconductor in dot form and toner adheres to them using the phenomena, two-color printing is allowed. Thus, a plurality of colors can be printed through a single optic system. Since these colors are simultaneously plotted, it is natural that no shift in the color printing position is produced.
As a light source suitable for such a laser beam printer, a semiconductor laser device having a small size and weight property is preferable. For example, a high-output semiconductor laser device disclosed in Japanese Patent Application Laid-Open No. 63(1988)-239891 is known as one example of such a light source. In this type of semiconductor laser device, a lens-shaped structure is provided within a wave-guiding path to prevent self filamentation. The high-output semiconductor laser device is materialized which performs both control on a higher lateral mode and astigmatism compensation and is oscillated in a fundamental mode.